I find things like that quite often myself. Usually I have to think about what else is in the area. Natural chert, Flakes? Cattle stomping on rocks?
Where I am tan / red / gray chert and obsidian is brought in, so I know it was handled and the breaks are clean and nothing very large is left behind.
But I do from time to time find that white stuff like you have there, in larger pieces...is it chert? (I'm not sure, it feels like it) I don't see it around naturally so I'm not so sure about it either as I never find it in fine flakes.
Good answer eh? I'll see if I have one around I'll take a picture...